1. Field of the Invention
This invention relates to an abrasive grain and a grindstone using plural number of the abrasive grain which performs self dressing function appropriate for grinding.
2. Discussion of the Background
Where a conventional grindstone grinds a workpiece made from iron material, ceramic material or etc., plural number of a hard abrasive grain, for example diamond or cubic boron nitride (CBN), is used for the grindstone. The grindstone is made from the steps of mixing the abrasive grains and vitrified bond or the like, and sintering the mixture. As shown in FIG. 5(a), a grinding wheel 100 is constructed from a core 103 and the grindstone (the abrasive grains 101 and the bond 102) glued on the circumference of the core 103. One part of each grain 101 crops out on the surface 100a of the grindstone and its surface is previously formed into microscopic roughness working as cutting edges 104 by a dresser, for example rotary diamond dresser (not shown).
Where the grinding wheel 100 includes the abrasive grains 101 whose diameter is comparative large (for example #80 to #120), each cutting edge 104 is worn and flattened due to the contact with the workpiece in the sequential grinding, thereby a flat surface 101a appears, as shown in FIG. 5(b). In the case of continuing to grind in such situation, because the grinding resistance increases between the workpiece and each surface 101a of the abrasive grains, grinding burn may occur and the grinding quality may deteriorate. To avoid the grinding burn, the dresser is used for deforming the flattened grains 101 to the microscopic roughness, thereby the cutting edges 104 are regenerated and the sequential grinding would be resumed.
However, where the grinding wheel 100 grinds the workpiece made from a hard material such as quenching steel, the abrasive grains 101 are remarkably worn. The grain 101 of diamond or CBN is too hard to perform self dressing function which regenerates the cutting edges 104 on its own surface due to the crack generated by the grinding load. Therefore, the grinding wheel 100 requires the dressing in advance and before the surface of the grain is flattened in the sequential grinding, thereby there may increase the cost related to the dressing.
Further, in case that the grinding wheel 100 has hard abrasive grains 101 and strong bond 102, the diamond grains of the dresser may be damaged during the dressing. Thus, the dressing condition must be set particularly.